This invention relates to wavelength-dispersive X-ray specrometer used in combination with a proportional counter tube.
Electromagnetic analysis is used to extract chemical information indicative of a substance through phenomena such as light emission, absorption, reflection, fluorescence, phosphorescence, scattering, diffraction and optical rotation, which occur as the result of an interaction between electromagnetic waves and the substance. Examples of electromagnetic analysis in which the wavelength region of the electromagnetic waves of interest is the X-ray region include fluorescent X-ray analysis (XRF) using X-rays as the source of excitation, EPMA using an electron beam narrowed down to less than one micrometer, and PIXE using a charged-particle beam such as a proton or helium-ion beam.
Characteristic X-rays, which are specific to an element, are emitted from a specimen excited by irradiation, and the X-rays are separated into spectral components by an analyzing crystal and then detected by a detector. A counting mechanism for counting the characteristic X-rays detected usually is provided with an amplifier for amplifying output pulses from the detector, a pulse-height selector (also referred to as a pulse-height analyzer) for selecting those amplified output pulses which have peak values within a specific range, and a counter for counting the output pulses selected by the pulse-height selector.
A gas-filled detector such as a proportional counter tube is used as the detector and operates when a high-voltage is applied thereto. With a detector of this kind, an increase in the strength of the entrant X-rays enlarges the amount of space charge within the detector so that there is a decrease in the effective detector voltage. This brings about a phenomenon referred to as an energy shift, in which there is a decrease in the peak value of the detector output pulse. When the energy shift occurs, the output pulses not selected by the pulse-height selector increase in number even if they are output pulses of the characteristic X-rays indicative of the element of interest, and hence there is an increase in the number of pulses which fail to be counted. This means that the relationship between X-ray intensity and pulse counting rate is no longer linear. The problem that results is a decline in analytical accuracy. In addition, the detector fluctuates due to a fluctuation in the high-voltage power supply and in accordance with the passage of time and temperature, and drift develops as a result.
Accordingly, it is necessary to apply a correction to the counting rate that has actually been measured. The simplest methods of applying this correction according to the prior art include a method which involves taking the energy shift into consideration and, if there is a shift in the pulse-height distribution, setting the window of the pulse-height selector to be large enough so that the entire distribution will fall within this window. Another simple method used when the range of intensities of measured X-rays is unspecified involves investigating the relationship between X-ray intensity and energy shift by actual measurement to obtain a correction function in advance, and automatically controlling the gain of the measurement system by this correction function when a specimen is measured. According to these methods, the window of the pulse-height selector is set to be large. A disadvantage which results is a corresponding increase in noise and a decline in measurement sensitivity. In addition, there is no assurance that the energy shift of the counter tube will be constant over an extended period of time, and the reliability of quantitative measurement is unsatisfactory.
Further, the conventional fluorescent X-ray spectrometer includes one in which the output end of the pulse-height selector is separately provided with a counting rate meter, and a high-voltage generator is controlled by the output signal from the counting rate meter in such a manner that the high voltage applied to the detector is corrected in conformity with X-ray intensity. In this conventional fluorescent X-ray spectrometer, the counting rate meter for controlling the high voltage of the detector is provided for output pulses which have passed through the pulse-height selector. As a consequence, all of the output pulses from the detector are not counted; only the output pulses of a target energy are counted.
On the other hand, the energy shift of a detector is dependent upon the strength of all X-rays that impinge upon the detector and is decided by the ionization count of the gas within the detector. Accordingly, though the high-voltage correction of the detector is applied with regard to X-rays of the target energy in the conventional fluorescent X-ray spectrometer, no correction is applied for X-rays of a non-essential energy. Consequently, in a case where there are many non-essential X-rays, a problem which arises is that the high-voltage correction of the detector is rendered inadequate and an improvement in analytical precision cannot be achieved.
A probe for fluorescent X-ray measurement having means for varying the magnitude of the high voltage supplied to the detector and adjusting the energy output of the detector to thereby maintain the stability of the apparatus has been disclosed in the specification of Japanese Patent Publication No. 52-21392. In this apparatus, the output end of an amplifier is provided anew with a high-level discriminator separate from a pulse-height selector in order to correct the high voltage of the detector with regard to all output pulses from the detector, and the high voltage applied to the detector is corrected by the output signal from the high-level discriminator. As a result, the apparatus has a more complicated mechanism and circuitry and the cost thereof is raised. In addition, a problem which remains unsolved is that the effects of the high-voltage correction due to higher order diffraction lines cannot be corrected for by the level discriminator.
Further, in the specification of Japanese Patent Application No. 59-191256 (Japanese Patent Application Laid-Open No. 61-68579) filed for patent by the present applicant on Sep. 11, 1984, there is disclosed an X-ray spectrometer which counts not only X-rays of a target energy but also X-rays of a non-essential energy to correct the high voltage applied to the detector and prevent the inconveniences caused by energy shift. This apparatus possesses two counting modes, namely a regular count for analytical counting and a preliminary count for correcting the applied voltage of the detector prior to the regular count.
In the preliminary counting mode, the upper-limit level of the pulse-height selector is raised and its lower-limit level is lowered by means for setting the level of the pulse height selector, all output pulses are counted, and the high-voltage applied to the detector is corrected by the set voltage value of high-voltage setting means stored in memory means as a memorized value extracted based upon the value of the count.
In the regular counting mode, the upper- and lower-limit levels of the pulse-height selector are returned to the normal levels for analytical counting and the high voltage corrected by the preliminary count is applied to the detector so that the latter may carry out X-ray detection. Though this method can be said to be an improvement on the above-described method of investigating the relationship between X-ray intensity and energy shift by actual measurement to obtain a correction function in advance, and automatically controlling the gain of the measurement system by this correction function when a specimen is measured, time is required in order to perform the preliminary count prior to the regular count, and another drawback is that real-time control cannot be carried out.
Accordingly, a first object of the present invention is to narrowly set the width of the window of a pulse-height selector, which selects the pulse height of output pulses from a proportional counter tube in an X-ray spectrometer, in conformity with the X-ray wavelength to be measured, and detect in real-time an energy shift in a measurement output when a specimen is measured, thereby making it possible to correct the measurement output.
A second object of the present invention is to reduce the number of output pulses not selected by a pulse-height selector owing to an energy shift in a measurement output of an X-ray spectrometer, thereby eliminating situations in which output pulses go uncounted and establishing a linear relationship between X-ray intensity and pulse counting rate so that the accuracy of analysis is improved.
A third object of the present invention is to compensate for a fluctuation in a high-voltage power supply and a fluctuation caused by the passage of time and a variation in temperature in an X-ray spectrometer.
A fourth object of the present invention is to perform an effective correction of the high voltage of a detector in an X-ray spectrometer even when there are many non-essential X-rays, thereby making possible an improvement in analytical accuracy.
A fifth object of the present invention is to construct an X-ray spectrometer of simple mechanisms and circuitry, in which a correction function is not obtained by interrupting a measurement in progress and previously investigating, through actual measurement, the relationship between X-ray intensity and energy shift.
Other objects and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.